https://www.recoilweb.com/measuring-the-overall-length-of-a-pistol-atf-has-altered-the-deal-151346.html?fbclid=IwAR2Jo9X91V1alRTaVgWox2shXvP10 CavGcmJnMEDjZ6vO14WbI2wigctiGk
"The letter states:
[Firearms Technology Industry Services Branch] FTISB has previously determined that “stabilizing braces” may be assembled on firearms as accessories… In contrast to stocks on rifles or shotguns…”stabilizing braces” are merely accessories and not relevant to the classification of a “pistol” under the statutory definition. That is, a folding stock on a rifle or shotgun is included in overall length measurements because the firearm must be “designed or redesigned….and intended to be fired from the shoulder” to be so classified. The stock is therefore an essential element in the statutory definition."
In the letter, ATF also specifies that:
[m]akers also create an artificial overall length measurement by attaching a folding stabilizing brace. Such a measurement would be problematic because the firearm could avoid classification as an “AOW,” yet retain the conceivability and remain fully functional. Measuring a folding (or telescoping) stabilizing brace would therefore undermine the comprehensive statutory and regulatory design of the GCA and NFA.…The measurement of a folding or collapsible stabilizing brace in the overall length of a firearm creates an artificial overall length that would permit a maker to avoid classification as an NFA “firearm” without a viable design purpose or legal justification.